The present invention relates to a process for the continuous thermal cracking of hydrocarbon oils.
For the thermal cracking of residual feedstocks--both long and short residues--two types of processes, namely furnace cracking and soaker cracking, are available. Furnace cracking implies that the actual cracking takes place in the last pipes of the furnace and to some extent in a transfer line which leads from the furnace outlet to a subsequent process stage. Residence times are not exactly known or controlled, but are short being of the order of one minute in the cracking zone. The pressure in the cracking zone varies to a great extent; it is high at the furnace inlet and quite low at the furnace outlet. In the case of soaker cracking, the feed is heated up to a suitable temperature and allowed to stay at that temperature for a period of usually 10-30 minutes in a vessel known as a soaker. A soaker is, hence, nothing more than a large empty unheated vessel which allows cracking to take place over a prolonged period. No heat is provided to the soaker and, since the cracking reaction is endothermic, the temperature of the oil drops by 10.degree.-30.degree. C. during the passage through the soaker.
Soaker cracking has basically the advantage of a significantly lower fuel requirement (hence, entailing the use of a smaller furnace) than is the case with furnace cracking. For this reason, a soaker is considered an attractive means of debottlenecking when furnace capacity is a limiting factor. U.S. Pat. No. 1,899,889 mentions a method for the thermal cracking of petroleum oils which comprises heating the oil in a series of tubes to a high temperature, introducing the hot feed into a digesting zone or soaking drum in which most of the cracking takes place and hence conducting the liquid and vapors into a fractionating zone, such as a bubble tower.
According to the above U.S. patent specification the hot feed is introduced into the lower portion of the soaking drum and the liquid and vaporous products leave through a common line at the upper portion of the drum.
In the process according to this U.S. patent specification an empty soaking vessel has been used.
We have found that at the same conversion of feed to gas plus gasoline the net amount of gas oil produced in soaker operation is somewhat higher than that obtained in furnace cracking.
However, the stability of the cracked residue is somewhat lower for soaker cracking than for furnace cracking at the same conversion levels.
It has now been found that the problem of the poorer stability of the cracked residue in case of soaker cracking can be solved.
According to the present invention a maximum conversion with a stable fuel as the heaviest of the products, is obtained by soaking the feed during an average residence time not shorter than 5 min. and not longer than 60 min. in a conversion zone which comprises at least two mixing stages. For a theoretical background of mixing stages see Perry, Chemical Engineers' Handbook, 3rd Edition, 1950, Section 17, page 1230.